Burning wood and related fuels efficiently in a domestic stove poses the dilemma of trying, on the one hand, to keep the burning temperature of the fire in the stove as high as possible to maximise the combustion efficiency, whilst upon the other hand, trying to get as much heat as possible from the stove into the room. A further challenge for a stove designer has been to provide for a practical and controllable method of burning combustible components carried in the smoke. Such a process is referred to as `burning its own smoke` and such components in some fuels can constitute up to fifty percent of the calorific value energy value of fuel burnt in the stove.
In a traditional burning arrangement, with smoke rising from the embers (wood or coal), the temperature of the smoke is not raised sufficiently high to reach the combustion temperature of at least some of the combustible components without the help of a catalyst in the smoke flow path. Such a catalyst is easily damaged irrevocably by components in the smoke arising from burning an unsuitable fuel such as, for example, painted wood.
Stoves have been produced and designed using what is termed a `down draughter` arrangement, whereby smoke is made to pass downwards, through glowing embers (such as of wood or coal) in a fire bed in the stove, before passing back to the flue. In this way the temperature of the glowing embers is sufficiently high for combustible components of the smoke to be burned. Few, if any, of these down draughter designs have proved both practicable and controllable and as a consequence such stoves have not been popular in the market place. This is despite their promise of much greater burning efficiency, with combustible elements in the smoke energy being converted to heat, and the concomitant benefit of the chimney not becoming coated with condensation products such as soot or tar.
U.S. Pat. No. 4,677,965 (Duerichen) shows a wood and coal burning heater which includes a primary combustion chamber for the controlled burning of a solid fuel positioned above a secondary combustion chamber for the subsequent combustion of combustible gases and pollutants which pass downwardly from the primary combustion chamber. An independent air supply is provided for each of the combustion chambers and air flowing to the primary combustion chamber is controlled to govern the rate of burn. A separate air supply is provided in the secondary combustion chamber and this air upon contact with the combustible gases and pollutants passing downwardly from the primary combustion chamber to cause further or secondary combustion to cleanse the smoke and gas of pollutants prior to discharge. Smoke and exhaust gases leaving the secondary combustion chamber pass first rearwardly and then upwardly along the back of the heater and then forwardly beneath the top surface of the heater prior to discharge to provide increased heat exchange contact between surfaces of the heater and exhaust smoke and gases.
The heater proposed and described by Duerichen is a complicated structure and does not make full use of air flows to promote effective operation of the stove when in use. In addition the flow passages described by Duerichen for the mixed exhaust gasses and what are referred to as `pollutants` would tend to be readily blocked so serving to adversely affect efficient operation of the stove.